1. Field of the Invention
The present invention relates generally to a sonar seeker head, and more particularly to a sonar seeker head employing active noise cancellation.
2. Description of the Background Art
Acoustic (i.e., sound) waves have long been used as a method of detecting objects undersea. Because acoustic waves are the type of waves that propagate best through water, they are the choice for applications such as undersea warfare (USW). Active sonar (i.e., Sound Navigation And Ranging), is an application of acoustic waves wherein direction and distance to a target may be obtained through the detection of reflected acoustic waves.
Sonar may be of two types, active or passive. Active sonar emits acoustic waves toward a target and picks up reflected waves to determine direction and distance. Passive sonar does not emit any acoustic waves, but only picks up acoustic waves emitted by the target. Passive sonar therefore has an advantage in that it is less likely to give away its own location. Passive sonar is often used when it is desired that the device not transmit any acoustic waves that might be used by the target to locate or track the emitting device, or even alert the target to the presence of the emitting device.
Sonar devices pick up undersea acoustic waves through the use of a transducer called a hydrophone. The hydrophone is capable of converting received acoustic waves into electrical signals that can be analyzed.
Sonar has practical application in the use of guidance of many types of marine vessels, including unmanned weapons such as torpedoes. A torpedo is essentially a warhead attached to a propulsion system and a guidance system. Without an effective guidance system, a torpedo is a blind missile. A sonar guidance system in the form of a seeker head is capable of detecting a target and guiding the torpedo to the target. The seeker head is capable of detecting target sound, whether it be reflected sound or sound emitted by the target (such as propulsion noise generated by the target).
FIG. 1 shows a first type of seeker head array that employs phase differences in received sound in order to determine the direction of origin of the sound source. The phased array seeker head is commonly located in a nose area of the torpedo 100 or other vessel, and is accompanied by amplifying and processing circuits. This torpedo seeker head configuration is generally constructed with a flat torpedo nose. Multiple hydrophone detection elements 103, typically configured in an array, are used in these acoustic torpedo seeker heads to observe phase differences. This seeker head observes differences in phase of the incoming acoustic signal as detected by the detection elements to determine direction of the target. For example, if the phase angle of the signal produced by the hydrophone elements on the right-hand side of the array is ahead of the phase angle of those on the left-hand side, the seeker head calculates that the target lies to the right of the axis of the seeker head. Conversely, if the phase angle of the signal produced by the hydrophone elements of the left hand side, upper side, or lower side are ahead of the phase angle of the hydrophones on the opposite side of the array, the derived xe2x80x9clookxe2x80x9d angle (the discerned direction of the target relative to the axis of the torpedo) indicates that the target lies to the left, above, or below the seeker head, respectively. The greater the difference in phase angle, the greater the look angle between the direction to the acoustic source and the axis of the seeker head.
FIG. 2 shows a hydrophone cluster 200 of another torpedo seeker head configuration that is capable of direction detection independent of frequency. This torpedo seeker head configuration is disclosed in U.S. Pat. 6,108,270 to DePoy, and is incorporated herein by reference. This frequency-independent seeker head includes three orthogonal directional hydrophones 203, 207, and 215, and one omni-directional hydrophone 212. This enables the hydrophone cluster 200 to find a sonic direction in three dimensions. The hydrophone cluster 200 may be located in a forward portion of a torpedo weapon, a submarine, a surface ship, or other marine vessel. In an alternate embodiment, the hydrophone cluster 200 may contain only two directional hydrophones and an omni-directional hydrophone, enabling a sonic direction to be found in two dimensions, or in three dimensions, if a non-resolvable between directions on either side of the plane of the two directional hydrophones can be admitted.
In the figure, orthogonal directional hydrophone 203 has a response pattern aligned with its axis Mxe2x80x94M, orthogonal directional hydrophone 207 has a response pattern aligned with its axis Nxe2x80x94N (orthogonal to axis Mxe2x80x94M), and orthogonal directional hydrophone 215 has a response pattern orthogonal to the response patterns of the other two directional hydrophones and aligned with an orthogonal vertical axis coming vertically out of the figure. Any suitable directional response pattern may be used for the orthogonal directional hydrophones 203, 207 and 215.
Hydrophone 212 is an omni-directional hydrophone, picking up acoustic signals in all directions. The omni-directional hydrophone 212 has a spherical response pattern with the omni-directional hydrophone 212 being located in the center of the sphere. An acoustic signal is received by the omni-directional hydrophone 212 at a constant phase and signal strength regardless of the directional position of the acoustic signal source in relation to the omni-directional hydrophone 212.
In accordance with the present invention, a look angle xe2x88x9d in a plane defined by any two directional hydrophones may be found by combining the outputs from two of the three hydrophones and using a phase from the omni-directional hydrophone 212. By using appropriate combinations of hydrophone pairs, look angles in all three dimensions may be found. The resulting look angles may be used to guide a torpedo or other such marine vessel.
A common problem for acoustic torpedo seeker heads or any type of sonar detector is xe2x80x9cownshipxe2x80x9d noise. Ownship noise has three principal components: screw noise, propulsion system machinery noise, and hydrodynamic noise.
Screw noise is the noise made by a turning propeller screw. At medium to high torpedo speeds, propeller noise occurs at medium to high acoustic frequencies, i.e., above the frequency passband of a seeker head operating at low frequency, hence screw noise need not be eliminated.
Torpedo machinery noise was extensively studied and measured during World War II. Machinery noise has been found to include mostly rather weak tonals occurring at low frequencies. Machinery noise is mostly independent of speed, and is chiefly structure borne. It occurs chiefly at low frequency, and is important at low speed, where other sources of low frequency noise are diminished. At higher speeds, such as at the speeds typically obtained by modern torpedoes, it is much weaker than hydrodynamic noise.
Hydrodynamic noise includes all noise resulting from the flow of water past the hydrophone, any hydrophone support structures, and the outer hull structure of the torpedo. It includes the turbulent pressures produced upon the hydrophone face in the turbulent boundary layer of the flow (flow noise), rattles and vibration induced in the hull plating, cavitation around appendages, and the noise radiated to a distance by distant vortices in the flow. Hydrodynamic noise increases strongly with speed, and because the origin of this noise lies close to the hydrophone, it is the principle source of noise at high speeds whenever the noise of propeller cavitation (itself a form of hydrodynamic noise) is insignificant.
A particular kind of hydrodynamic noise has been termed flow noise. Flow noise consists of the pressures impinging upon the hydrophone face created by turbulent flow. Although these turbulent pressures are not true sound, in that they are not propagated to a distance, they form what has been termed pseudosound and may give rise to a fluctuating noise voltage at the output of a pressure hydrophone.
FIG. 3 illustrates the mechanism of flow noise. As the water flows around the torpedo 100, at some point the flow separates from the torpedo 100 and becomes turbulent. The turbulent flow may create forces on the torpedo, and which may be formed of the force components Fx, Fy, and Fz, as shown.
Flow noise does not vary in frequency with vessel speed. Rather, it varies markedly in amplitude with speed, while the sonic noise strength varies inversely with frequency at a rate of approximately 9 decibels (db) per octave.
FIG. 4 shows measured flow noise levels at a variety of speeds and for two hydrophone sizes. The figure is a result of a study of flow noise in The Physics of Flow Noise, G. P. Haddle and E. J. Skudrzyk, J. Acoust. Soc. Amer., vol. 46, 1969, incorporated herein by reference. The nose of the torpedo 100 passing through the water gives the water a component of velocity away from the torpedo, thus causing a low pressure volume alongside the torpedo, into which the water returns in a turbulent fashion. This turbulence pounds the aft end of the torpedo 100, thereby inducing low frequency noise in the torpedo 100. This low frequency noise travels forward through the structure of the torpedo 100. The resultant vibration in the seeker head at the forward end of the torpedo 100 causes a vibration interaction between the hydrophone and the water. This interaction, i.e., flow noise, is impossible to distinguish from noise in the water itself, and interferes with target detection.
If the principal operating frequency of the torpedo seeker head is at a medium to high frequency, the effects of flow noise are minimized. Alternatively, if the principal operating frequency of the torpedo seeker head is at low frequency, the effects of screw noise are avoided. Machinery noise, being significantly weaker than flow noise at most torpedo speeds, can be neglected.
Therefore, there is a need for a means of discriminating against, or wholly or partially canceling, ownship noise, in order to enhance the in-band signal-to-noise ratio at the hydrophones, and hence to enhance the seeker head performance. A directional hydrophone seeker head, while able to operate at any frequency, is able to operate at low frequency. Operation at low frequency is desirable because the anechoic coatings applied to most, if not all, combat submarines today are ineffective or non-operative at low frequency.
The directional hydrophone configuration of a torpedo seeker head, as shown in FIG. 2, may avoid the effects of ownship screw noise and ownship propulsion machinery noise. This is because such a configuration is independent of frequency and may look at frequency bands outside of the screw noise and machinery noise frequency bands. However, this type of seeker head will be affected by flow noise.
What is needed, therefore, are improvements in torpedo seeker heads to reduce flow noise.
A noise cancellation system adapted for use with a seeker head of a marine vessel is provided according to one embodiment of the invention. The seeker head employs one or more hydrophones that generate one or more output signals. The noise cancellation system comprises a plurality of motion sensors. The plurality of motion sensors generate a plurality of noise signals based on a hydrodynamic flow noise caused by the marine vessel and acting on the marine vessel. The noise cancellation system further comprises a processor receiving the plurality of noise signals and the one or more output signals and applying an active noise cancellation to the one or more output signals to substantially cancel out the flow noise. The active noise cancellation is based on the noise signal.
A hydrodynamic flow noise reduction shroud adapted for use with a torpedo is provided according to one embodiment of the invention. The shroud comprises a substantially ring-shaped shroud extending between a leading edge and a trailing edge and includes a front aperture and a rear aperture. The shroud is adapted to be affixed over a nose portion of the torpedo. The shroud further comprises a plurality of pylons adapted to affix the shroud in a spaced-apart relation from the nose portion.
A method of noise cancellation for one or more output signals generated by one or more hydrophones of a seeker head of a marine vessel is provided according to one embodiment of the invention. The method comprises the step of generating a noise signal based on a hydrodynamic flow noise caused by the marine vessel and acting on the marine vessel. The method further comprises the step of applying an active noise cancellation to the one or more output signals to substantially cancel out the flow noise. The active noise cancellation is based on the noise signal.
A method of noise cancellation for a seeker head of a marine vessel is provided according to another embodiment of the invention. The noise cancellation is provided for one or more output signals generated by one or more hydrophones of the seeker head. The method comprises the step of generating a noise signal based on a hydrodynamic flow noise caused by the marine vessel and acting on the marine vessel. The method further comprises the step of mechanically moving the seeker head. The method further comprises the step of electronically subtracting the noise signal from the one or more output signals. The moving and subtracting are based on the noise signal and substantially cancel out the flow noise.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.